This invention relates generally to telephone communications systems and more particularly, it relates to a data station termination circuit for connecting a four-wire transmission facility to either a 4-wire or 2-wire subscriber station. The termination circuit has particular use in voice or audio band data channel applications for interfacing between a central office having a 4-wire circuit and a customer station having either a 4-wire or 2-wire terminating equipment such as a modem.
In telephony systems, a termination circuit is used to connect unidirectional receive and transmit paths of a 4-wire line at a central office with a 4-wire or 2-wire subscriber station. In the case of a 2-wire subscriber station, the termination circuit must separate bidirectional signals at a subscriber port so as to amplify or attenuate the signals in each direction separately. Further, the termination circuit must prevent the central office-transmitted signals received at the transmit-in port from being returned to the central office via the transmit-out port. Typically, prior art termination circuits provided a gain network formed of a first amplifier with a resistor network in the receive path and a loss network formed of a second amplifier with associated resistors in order to perform gain and attenuation functions. These prior art termination circuits thus suffer from the disadvantage of utilizing a relatively large number of circuit components and thus increased manufacturing costs.
It would be desirable to provide a data station termination circuit for connecting a 4-wire, voice frequency transmission facility to either a 4-wire or 2-wire subscriber station in a telephone communication system for effecting a four-wire or two-wire mode of operation which includes a gain/loss network formed of a single operational amplifier, a plurality of binary weighted resistors, and a plurality of separately operated single-pole, single-throw switches capable of performing both the gain and loss functions in the receive path. It would also be expedient to provide a data termination circuit which can be operated from either line power or shelf power. Further, a battery back-up is provided to permit its operation during loss of line power or shelf power and allows disconnecting of the line power current path during a loopback operation. An optional plug-in equalization circuit may be added in the transmit path if desired. A single ended cancellation operational amplifier is used to prevent the central office-transmitted signals received at the receive-in port from being returned thereto.